Link/Join two products

Hi,

I was wondering if anyone has found a way to link two products together? As in: Adding product A to the cart will automatically add product Z aswell, depending on circumstances.

Eg, Products are software/images, so physical media (CD/DVD) is required. Adding more and more software/images until CD is full and another CD is required..

Im not having googling with the words "link" and "join" !!! 

Any help would be much appreciated,
wulf